1. Field of the Invention
This invention relates to the field of oil well drilling, in which a drilling rig is supported by a traveling block. The traveling block is carried by a cable which is looped around a crown pulley and attached to a rotating drum assembly. Specifically, the invention relates to a system for accurately determining the position of the traveling block. This data can be used to determine penetration of the oil well drill bit, the rate of penetration, the hole depth, and speed of the traveling block.
2. Description of the Prior Art
The most common method of obtaining desired positional measurements in a drilling rig is to utilize a measuring line which directly measures distance. These devices have problems with respect to reliability as well as difficulty of repair. As an alternative to direct measurement by means of a wire line, traveling block position may be obtained by determining the amount of cable paid out or wound onto the rotating drum from an arbitrary reference point. This amount of cable is mathematically related to the position of the traveling block in the oil derrick above the rig floor. This relation is a function of the number of feet of cable paid out (or wound in) per drum revolution and the number of lines strung between the crown block and traveling block. A system employing measurement of drum rotation to determine traveling block position is shown in U.S. Pat. No. 4,156,467 to Patton et al.
Because different layers of cable wound around the drum have different diameters, the amount of cable paid out during each revolution of the drum will not be constant. In order to increase the accuracy of the block position determination, some compensation must be made for varying diameters of wraps about the drum. The Patton patent uses a computer to accomplish this function. Other measurement devices which utilize electronics to compensate for varying diameters of material wound about a drum are shown in U.S. Pat. Nos. 3,710,084 to Slagley et al and 4,024,645 to Giles. Although such systems may provide accurate measurements, the use of an electronic measurement system is not desirable in an oil rig environment, because of the possibility of stray electrical signals affecting the operation of the system.
The use of pneumatic devices to measure movement of the traveling block is shown in U.S. Pat. Nos. 3,750,480 to Dower and 3,214,762 to Van Winkle. The use of pneumatic devices avoids problems of interference associated with electronic measurement systems. However, the Dower and Van Winkle systems do not incorporate any type of compensation scheme to ensure that accurate measurements are provided despite the varying diameters of cable wraps around the drum.